Lightweight cast-iron product

ABSTRACT

Provided is a cast-iron product that satisfies demands for both weight reduction and strength enhancement with good balance. A cast-ion product having an inner honeycomb structure is produced by a process including: a decarburization step, in which a cast-iron product made of a hypoeutectic cast iron is heated to form a decarburized layer on the surface of the cast-iron product; an outflow-hole formation step, in which an outflow hole penetrating through the decarburized layer into an inner region is formed; and a liquation step, in which the cast-iron product is heated to a temperature lower than the melting point of the decarburized layer and higher than the melting point of the hypoeutectic cast iron remaining inside, while being held in such a manner that the outflow hole is located in the lower portion thereof.

TECHNICAL FIELD

The present invention relates to a lightweight cast-iron product and amethod for producing the same.

BACKGROUND ART

A number of cast-iron products are currently used for the main bodiesand parts of automobiles or other mid-size to large-size mechanicalsystems. As compared to plastic products, cast-iron products are moresuitable for such parts or portions that must have high mechanicalstrength or heat resistance. As compared to other kinds of metalsincluding steels, they are more suitable for such parts which havecomplex shapes or which need to be inexpensively produced. Accordingly,cast-iron products are used in a large number of areas where theaforementioned properties and performance are required. However, theirsuperiorities are gradually undermined by plastic materials and otherkinds of metals which have achieved technical improvements in variousfunctionalities. Under such a situation, similar technical improvementsof cast-iron products for meeting the demands of the times are alsoexpected.

One of the major areas where the cast-iron products are used is theautomobile industry, where the reduction of weight has always been acritical problem. Needless to say, there are also other applicationareas in which weight reduction is regarded as an eternal problem. Toaddress this issue, the present inventor and his associates haveprovided an epoch-making solution, i.e. the hollow cast-iron product(Patent Document 1).

BACKGROUND ART DOCUMENT Patent Document

-   Patent Document 1: JP-B 4099535

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

The cast-iron product described in Patent Document 1 has achievedsignificant progress in terms of weight reduction. As for its strength,the product is adequately stronger than plastic products. However, dueto its hollow structure, the product is often not strong enough tosubstitute for such parts that have conventionally been made of castiron.

Accordingly, the problem to be solved by the present invention is toprovide a cast-iron product which satisfies demands for both weightreduction and strength enhancement with good balance.

Means for Solving the Problems

The lightweight cast-iron product according to the present inventionaimed at solving the aforementioned problem includes a shell portionhaving a melting point increased by surface decarburization, and ahoneycomb region inside the shell portion.

A method for producing such a cast-iron product includes:

a) a decarburization step, in which a cast-iron product made of ahypoeutectic cast iron is heated to form a decarburized layer on thesurface of the cast-iron product;

b) an outflow-hole formation step, in which an outflow hole penetratingthrough the decarburized layer into an inner region is formed; and

c) a liquation step, in which the cast-iron product is heated to atemperature lower than the melting point of the decarburized layer andhigher than the melting point of the hypoeutectic cast iron remaininginside, while being held in such a manner that the outflow hole islocated in the lower portion thereof.

The aforementioned “hypoeutectic cast iron” is a cast iron having acomposition whose carbon equivalent CE satisfies the following equation(I):

2.0<CE=C %+(Si %+P %)/3<4.3  (1)

where the “%” values are in weight percent.

If CE is within this range, the melting point of the hypoeutectic castiron will be approximately 1147° C., regardless of the CE value.Accordingly, it is preferable to set the heating temperature in step c)at a value higher than that melting point (1147° C.) by up toapproximately 50° C., i.e. within a range from 1147 to 1200° C. As forthe carburized layer, if its carbon concentration is equal to or lessthan 1%, the melting point of that portion (the carburized layer) willbe approximately 1350° C.

Effect of the Invention

The previously described process according to the present invention isbasically similar to the process described in Patent Document 1.However, a significant difference exists: The cast-iron part accordingto Claim 1 of Patent Document 1 is merely described as a “cast-ironpart” and there is no specific limitation on its material. By contrast,the present invention is limited to a product “made of a hypoeutecticcast iron.” It should also be noted that the cast-iron part according toClaim 3 of Patent Document 1 is characterized by “having a eutecticcarbon concentration.” In this respect, the invention described inPatent Document 1 is entirely different from the method according to thepresent invention.

As described previously, the present invention uses a hypoeutectic eastiron as the material. As can be understood from the phase diagram inFIG. 1, the hypoeutectic cast iron in the non-decarburized portion(inner region) does not completely melt in the Equation step c); aportion of the dendrite-shaped γ phase, which has a higher meltingpoint, remains in a lattice form. As a result, the inner region(non-decarburized portion) will have a honeycomb structure, unlike thecast-iron part described in Patent Document 1 whose inner region iscompletely hollow. Naturally, the honeycomb structure is somewhatinferior to the complete hollow structure in terms of theweight-reducing effect. However, it is adequately stronger than thehollow structure. Thus, by the method according to the presentinvention, it is possible to produce a cast-iron product which satisfiesdemands for both weight reduction and strength enhancement with goodbalance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a phase diagram of the iron-carbon system (for Si=0%).

FIG. 2 is an external view of a cast-iron product 1 as the firstexample.

FIG. 3 is a table showing the major chemical composition of thecast-iron product 1 of the first example.

FIG. 4 is a sectional view of the cast-iron product 1 after ahoneycomb-structure formation process.

FIG. 5 is a table showing the major chemical composition of a cast-ironproduct 2 as the second example.

BEST MODE FOR CARRYING OUT THE INVENTION

Two examples of the cast-iron product according to the present inventionwill be hereinafter described.

FIRST EXAMPLE

The first example is a bearing cap as shown in FIG. 2. Its chemicalcomposition was as shown in FIG. 3. As indicated in the last field ofFIG. 3, the material of the cast-iron product of the present example(which is hereinafter called the “cast-iron product 1”) has a CE valueof 4.03, and therefore, is hypoeutectic.

A decarburization process was performed as follows: The cast-ironproduct 1 was heated to 1060° C. in an electric furnace and maintainedat that temperature for 12 hours in an ambience of a converted gasprepared from a city gas by altering its composition toCO/(CO+CO₂)*100-75%. Following the decarburization process, the ambientgas was replaced with N₂ gas and the furnace was cooled to 500° C.,after which the product was cooled in atmospheric air.

After the cast-iron product 1 was cooled to room temperature, a hole of3 mm in depth (outflow hole) was bored in one portion of the product(indicated by “A” in FIG. 2) with a drill of 4.5 mm in diameter.

Then, the cast-iron product 1 was placed in the same electric furnace,with the outflow hole directed downward. After the inner space of thefurnace was filled with N₂ gas, the temperature was increased to 1185°C. in 50 minutes and maintained at that temperature for eight minutes.When the cast-iron product 1 reached that temperature, the molten metalbegan to flow out of the outflow hole. A few minutes later, the outflowof the molten metal began to slow down, which was almost stopped at theend of the eight-minute period. Then, the product in the furnace wascooled to 500° C. in the same N₂ ambience, after which it was cooled inatmospheric air.

After being cooled to room temperature, the cast-iron part 1 was cut ata plane in the middle of its thickness. FIG. 4 shows the cross section.As shown, a honeycomb structure was formed inside the decarburizedperipheral layer which had a thickness of approximately 3 mm.

The weight of the cast-iron part 1, which was 370 gram before thepreviously described process, decreased to 315 gram after the process.The weight reduction rate was 15%.

SECOND EXAMPLE

The second example is an oil pump cover. The cast-iron product in thepresent example (which is hereinafter called the “cast-iron product 2”)is a disc-shaped part of 185 mm in outer diameter having a45-mm-diameter hole at its center. The central hole was surrounded by acircumferential rim with a thickness of 28 mm. The outer circumferentialportion of the disc was 21 mm in thickness. Before undergoing theprocess, this cast-iron product 2 weighed 3.45 kg, which wasapproximately nine times the weight of the cast-iron product 1. Thechemical composition of the used material was as shown in FIG. 5. Its CEvalue was 4.11 (hypoeutectic).

The decarburization of the cast-iron product 2 was performed in the samemanner as the previous example. That is to say, it was heated to 1060°C. in an electric furnace and maintained at that temperature for 12hours in an ambience of a converted gas prepared from a city gas byaltering its composition to CO/(CO+CO₂)*100=75%. Following thedecarburization process, the ambient gas was replaced with to N₂ gas andthe furnace was cooled to 500° C., after which the product was cooled inatmospheric air. In the obtained cast-iron product 2, two outflow holeswere bored in the circumferential portion with a drill of 8 mm indiameter.

Then, the cast-iron product 2 was placed in the same electric furnace,with the outflow holes directed downward. After the inner space of thefurnace was filled with N₂ gas, the heating temperature was increased to1190° C. in 60 minutes and maintained at that temperature for 12minutes. Similar to the previous example, when the cast-iron product 2reached that temperature, the molten metal began to flow out of theoutflow holes. The outflow was almost stopped at the end of the12-minute period. Then, the product in the furnace was cooled to 500° C.in the same N₂ ambience, after which it was cooled in atmospheric air.

After being cooled to room temperature, the cast-iron part 2 was cut ina radial direction, and its inner condition was visually checked. Thischecking confirmed that a honeycomb structure was formed both inside therim surrounding the central hole and inside the outer circumferentialportion having the increased thickness. After the hollow-structureformation process, the weight of the cast-iron part 2 was 2.31 kg. Theweight reduction rate was 33%.

1. A lightweight cast-iron product comprising a shell portion having amelting point increased by surface decarburization and a honeycombregion inside the shell portion.
 2. The cast-iron product according toclaim 1, wherein a a carbon equivalent of a composition of a cast-ironbefore the surface decarburization is hypoeutectic.
 3. A method forproducing a lightweight cast-iron product, comprising: a) adecarburization step, in which a cast-iron product made of ahypoeutectic cast iron is heated to form a decarburized layer on asurface of the cast-iron product; b) an outflow-hole formation step, inwhich an outflow hole penetrating through the decarburized layer into aninner region is formed; and c) a liquation step, in which the cast-ironproduct is heated to a temperature lower than a melting point of thedecarburized layer and higher than a melting point of the hypoeutecticcast iron remaining inside, while being held in such a manner that theoutflow hole is located in a lower portion thereof.